The R-T-B based sintered magnet (R represents a rare earth element, T represents one or more elements of the iron group with Fe as an essential element, and B represents boron), a representative of which is Nd—Fe—B based sintered magnet, is advantageous for miniaturization and high efficiency of the machines used due to high saturation magnetic flux density, and thus can be used in a voice coil motor of a hard disk drive, etc. Recently, it is also suitable for use in various industrial motors or drive motors of hybrid vehicles, etc., and it is desired to be further popularized in these fields from the viewpoint of energy conservation, etc. However, when applied in the hybrid vehicles and the like, the R-T-B based sintered magnet will be exposed to a high temperature, and thus the suppression on demagnetization at high temperature caused by heat becomes important. Regarding said suppression on demagnetization at high temperatures, it is well known that the method of increasing the coercivity (Hcj) at room temperature of an R-T-B based sintered magnet is effective.
For example, as a method for increasing the coercivity (Hcj) at room temperature of an Nd—Fe—B based sintered magnet, a method using heavy rare earth element such as Dy, Tb to replace part of Nd in the compound Nd2Fe14B which acts as the main phase is well-known. By replacing part of Nd with heavy rare earth element, the magneto-crystalline anisotropy can be increased, and consequently, the coercivity at room temperature of the Nd—Fe—B based sintered magnet may be increased sufficiently. In addition to the replacing method with heavy rare earth element, it is also effective to add elements such as Cu in increasing the coercivity at room temperature (Patent Document 1). It is considered that by adding of the Cu element, the Cu element forms, e.g., an Nd—Cu liquid phase, at the grain boundary, and hence the grain boundary becomes smooth, inhibiting nucleation of reverse magnetic domains.
In another aspect, Patent Document 2 and Patent Document 3 have disclosed the technology for enhancing the coercivity by controlling the grain boundary phases which act as the microstructure of a rare earth based magnet. It may be derived from the drawings in these patent documents that, the grain boundary phases as mentioned herein refer to grain boundary phases surrounded by three or more main-phase crystal grains, i.e., triple junctions. Patent Document 2 has disclosed a technology for constructing two kinds of triple junctions with different Dy concentrations. That is, it has disclosed that by forming a part of grain boundary phases (triple junctions) with higher Dy concentration without increasing the entire Dy concentrations, a high resistance to the reversal of the magnetic domain can be provided. Patent Document 3 has disclosed such a technology in which, three, i.e., first, second and third grain boundary phases (triple junctions) which are different in total atomic concentrations of rare earth element is formed, the atomic concentration of rare earth element of the third grain boundary phases is lower than that of the other two kinds of grain boundary phases, and in addition, the atomic concentration of the Fe element of the third grain boundary phases is higher than that in the other two grain boundary phases. As a result, third grain boundary phases containing a high concentration of Fe are formed among the grain boundary phases, which can induce the effect of increasing the coercivity. Further, Patent Document 4 has disclosed a R-T-B-based rare earth-based sintered magnet which is consisted of a sintered body having a main phase mainly containing R2T14B and grain boundary phases containing more R than the main phase, with said grain boundary phases comprising: a phase with the total atomic concentration of rare earth element being 70 at % or more and a phase with the total atomic concentration of the above-mentioned rare earth element being 25 to 35 at %. It also has disclosed that the above-mentioned phase with the total atomic concentration of the rare earth element being 25 to 35 at % is named a transition metal-rich phase, and the atomic concentration of Fe in said transition metal-rich phase is preferably 50 to 70 at %. As such, the effect of increasing the coercivity is achieved.